Regarding the positional accuracy of map information provided by a topographic map on a scale of 1:25,000 issued by Geospatial Information Authority of Japan, it is said that the position may be erroneous by a dozen meters due to human error, measurement error, or the like. Also, it is known that road elevation is greatly influenced if the position of the tunnel entrance is off-point because, in general, a topographic elevation widely changes near the tunnel entrance. In other words, when the position of the tunnel entrance is accidentally recognized at a position slightly displaced toward the tunnel, the elevation at the slope of the mountain located above the tunnel is erroneously computed as the road elevation of the entrance.
When the above erroneous elevation is used, for example, the road gradient within the tunnel, which is computed by using the elevation, may generate such error.
Thus, in order to highly accurately estimate the road elevation, a vehicular road elevation estimating device is proposed (Patent Document 1). The vehicular road elevation estimating device computes a reliability of a surface elevation value that is computed from surface elevation data. Then, the vehicular road elevation estimating device executes a filtering process for weighing the data with higher reliability to estimate the road elevation of a value closer to an actual road elevation.
Please refer to Patent Documents 2 to 4 that disclose conventional techniques related to the present invention.